Magnitude and source area estimations of severe prehistoric earthquakes in the western Austrian Alps

In: Natural hazards and earth system sciences: NHESS, Band 22, Heft 6, S. 2057-2079

Abstract

Abstract. In slowly deforming intraplate tectonic regions such as the Alps only
limited knowledge exists on the occurrence of severe earthquakes, their
maximum possible magnitude, and their potential source areas. This is mainly
due to long earthquake recurrence rates exceeding the time span of
instrumental earthquake records and historical documentation. Lacustrine
paleoseismology aims at retrieving long-term continuous records of seismic
shaking. A paleoseismic record from a single lake provides information on
events for which seismic shaking exceeded the intensity threshold at the
lake site. In addition, when positive and negative evidence for seismic
shaking from multiple sites can be gathered for a certain time period,
minimum magnitudes and source locations can be estimated for
paleo-earthquakes by a reverse application of an empirical intensity
prediction equation in a geospatial analysis. Here, we present potential
magnitudes and source locations of four paleo-earthquakes in the western
Austrian Alps based on the integration of available and updated lake
paleoseismic data, which comprise multiple mass-transport deposits on
reflection seismic profiles and turbidites and soft-sediment deformation
structures in sediment cores. The paleoseismic records at Plansee and
Achensee covering the last ∼10 kyr were extended towards the
age of lake initiation after deglaciation to obtain the longest possible
paleoseismic catalogue at each lake site. Our results show that 25 severe
earthquakes are recorded in the four lakes Plansee, Piburgersee, Achensee,
and potentially Starnbergersee over the last ∼16 kyr, from
which four earthquakes are interpreted to have left imprints in two or more
lakes. Earthquake recurrence intervals range from ca. 1000 to 2000 years,
with a weakly periodic to aperiodic recurrence behavior for the individual
records. We interpret that relatively shorter recurrence intervals in the
more orogen-internal archives Piburgersee and Achensee are related to
enhanced tectonic loading, whereas a longer recurrence rate in the more
orogen-external archive Plansee might reflect a decreased stress transfer
across the current-day enhanced seismicity zone. Plausible epicenters of
paleo-earthquake scenarios coincide with the current enhanced seismicity
regions. Prehistoric earthquakes with a minimum moment magnitude (Mw)
5.8–6.1 have occurred around the Inn valley, the Brenner region, and the
Fernpass–Loisach region and might have reached up to Mw 6.3 at
Achensee. The paleo-earthquake catalogue might hint at a shift in severe
earthquake activity near the Inn valley from east to west to east during
postglacial times. ShakeMaps highlight that such severe earthquake scenarios do not solely impact the enhanced seismicity region of Tyrol but widely affect
adjacent regions like southern Bavaria in Germany.